A power converter is a power processing circuit that converts an input voltage or current waveform into a specified output voltage or current waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage into a specified output voltage. A flyback converter is one example of an isolated switched-mode power converter that is typically employed in applications wherein a stable, regulated voltage is desired at the output of the power converter.
A flyback converter generally includes a switching circuit having at least one power switch, an isolation transformer, a rectifier having a freewheeling diode, and an output filter. A controller is included and employed to control the power switch. The conventional flyback converter operates as follows. The power switch is intermittently switched to apply the input voltage across a primary winding of the isolation transformer. The rectifier is coupled to a secondary winding of the isolation transformer and regulates the voltage received therefrom. The output filter smooths and filters the rectified voltage to provide a substantially constant output voltage at the output of the flyback converter. The controller monitors the output voltage and adjusts the duty cycle of the power switch to maintain the output voltage at a constant level despite variations in the input DC voltage and the load current. Depending on the duty cycle of the power switch, the output voltage may be regulated to any desired voltage between zero and the input voltage.
Analogous to other types of power converters (e.g., forward converter, half-bridge converter), the flyback converter is subject to inefficiencies that impair its overall performance. More specifically, the power switch, which may be a metal-oxide semiconductor field-effect transistor (MOSFET), has a finite switching speed and an intrinsic parasitic capacitance that affect its performance. A charge built up in the parasitic capacitance of the power switch may be dissipated during turn-on of the power switch, resulting in switching loss. Further, the freewheeling diode also has a finite switching speed and is, therefore, subject to a reverse recovery phenomenon that may induce a substantial current spike through both the power switch and the freewheeling diode. The losses associated with the power switch and the freewheeling diode increase linearly as the switching frequency of the converter is increased. Therefore, minimizing the reverse recovery phenomenon and the switching loss associated with the freewheeling diode and power switch will improve the overall efficiency of the flyback converter.
Accordingly, what is needed in the art is an active clamp, employable with a variety of power converter topologies, that reduces the losses associated with the reverse recovery phenomenon and further reduces the switching loss associated with the power switch of a power converter.